This invention relates to a method and an apparatus for continuous separation of suspended material from fluids. The apparatus can be used for said purposes when the fluid, which usually is water, continuously is fed to the apparatus and the suspended material is either in the fed fluid flow or entirely or partly through addition of chemicals produced in the apparatus itself. Two flows are continuously obtained from the apparatus, hereinafter called the clear water flow and the slurry flow. The clear water flow contains the major part of the fed fluid and only an insignificant part of the suspended material. The major part of the suspended material is in the slurry flow which has a proportionately high dry matter content.
This kind of separation can be achieved by sedimentation in case the density of the suspended material is higher than the density of the fluid. In the opposite case the separation takes place by flotation. If there exists a difference in density between the suspended material and fluid, separation can also be achieved with the aid of centrifugal force. Filtration is another common method for separation of suspended material from fluid flows. The separation mechanisms have been used in prior known apparatuses such as sedimentation basins, flotation apparatuses, centrifuges, cyclones, and fluid filters.
In spite of the variety of apparatuses that exist for separating suspended material from fluid flows, it has become apparent that none of the prior known apparatuses is without considerable drawbacks in special cases. Such cases are found e.g. when purifying heavy waste water flows in which the particle size of the suspended material is small and its density almost equals that of water. As an other example may be mentioned the recovery of fibres from the white water system of paper-making machines. In such cases effective separation of the suspended material from the fluid flow, with the aid of sedimentation, requires long retention periods for the fluid in the sedimentation apparatus which then attains uneconomically large dimensions. By shortening the sedimentation route by fixed built-in devices of different types the retention period can be shortened to a certain extent. However, the slurry flow is taken out of the sedimentation apparatuses by pumping together with the fluid which causes the dry matter content of the slurry to be low and usually it has to pass secondary dewatering in another apparatus e.g. a centrifuge or a band filter press in order to be handled as solid material.
Suspended particles can be separated from a fluid by filtering irrespective whether there exists a difference in the density of the particles and fluid or not. In a filter the slurry flow is usually removed above the free fluid surface in the apparatus. Therefore the slurry has a relatively high dry matter content. In a filter the ensuing filter cake constitutes the filtering medium. This restricts the possibilities to use filters to separate suspended material from large fluid flows as the ensued filter cake being microporous and, in addition to that, compressible, constitutes a great flow resistance to the fluid. As the entire fluid flow shall pass the filter cake, unrealistic large filter surfaces are required, from the economic point of view, if the fluid flow is big.
Through the Swedish Pat. No. 333,721 it is known a device for separation of suspended material from a fluid by means of flow channels for the suspension with channel walls mobile in the longitudinal direction of the channel for bringing forth a velocity gradient in the suspension, whereby the suspended material tends to migrate towards areas in the suspension where the velocity gradient is less so that a clear solution and a thickened solution is received.
This known device has, however, a far too low capacity for any practical use. In order to provide a velocity gradient the fluid flow should be laminar, which means that the flowing velocity remains so low that one cannot achieve any in practice useful separation capacity with this device.
The object of the present invention is thus to provide a method and a device for separation of suspended material from a fluid flow, in which the former disadvantages have been eliminated and with a good capacity.